A claw pump has a housing forming as pump chamber and two claw-type rotors rotating in the housing in opposite directions to each other in a non-contact manner with a very narrow clearance kept between the rotors. The two claw-type rotors form to compression pocket, and a gas compressed in the compression pocket is discharged through a discharge opening. Suction, compression and exhaust are performed continuously without using either a lubricant or sealing liquid to create a vacuum condition or pressurized air. Because no lubricant or other liquid is used, it is possible to achieve clean evacuation and discharge. In addition, the claw pump has the following advantages. It is possible to realize a higher compression ratio than Roots pumps, which have no compression process. Because the rotors rotate in a non-contact manner, it is possible to readily realize energy-conservation pumping according to need by controlling the number of revolutions.
Patent Document 1 discloses the structure of such a claw pump. The inventors of the present invention have already proposed a multistage vacuum pump capable of suppressing pulsation and power fluctuation, using a claw pump (Patent Document 2).
FIG. 8 illustrates the structure of a conventional vacuum claw pump. In FIG. 8, a claw pump 100 has a pump chamber 102 comprising an outer peripheral wall 104 having an inner surface with a sectional configuration defined by two mutually overlapping circles. The pump chamber 102 further comprises two (front and rear) side walls 106 juxtaposed to the outer peripheral wall 104. It should be noted that the front side wall is not shown in the figure. Two rotating shafts 108a and 108b extend through the pump chamber 102 in parallel to each other. The rotating shaft 108a has a male rotor 110 secured thereto. The male rotor 110 has two radially projecting claw portions 112a and 112b. The rotating shaft 108b has a female rotor 114 secured thereto. The female rotor 114 has recesses 116a and 116b into which the claw portions 112a and 112b enter, respectively.
A suction opening 118 is provided on one side of a plane L containing the axes of the rotating shafts 108a and 108b, and a discharge opening 120 is provided on the other side of the plane L. Minute clearances are provided between the pair of rotors and between each rotor and the wall surface of the pump chamber 102. If the clearances are excessively large, back flow occurs in the pump chamber, causing a reduction in efficiency. On the discharge opening side of the plane L, a compression pocket P is formed being surrounded by the pair of rotors 110 and 114, the outer peripheral wall 104 and the side walls 106. As the male and female rotors 110 and 114 rotate in the directions of the arrows, respectively, the volume of the compression pocket P decreases progressively, and the gas in the compression pocket P is compressed correspondingly. At the time shown in FIG. 8(B), the discharge opening 120 is in communication with the compression pocket P, and the gas in the compression pocket P is discharged through the discharge opening 120.
At the time shown in FIG. 8(C), the opening area of the discharge opening 120 is reduced by the rotation of the male and female rotors 110 and 114, and the compression pocket P is also reduced. At the time shown in FIG. 8(D), the discharge opening 120 is closed, while the compression pocket P still exists. FIG. 8(D) shows that the compression is continued in the compression pocket P. In the final stage of the compression process, the volume of the compression pocket P becomes zero; therefore, the volume ratio (compression ratio) becomes infinite in terms of calculation.